1. Field of the Invention
The present invention relates to heat pipes, and more particularly to a monogroove heat pipe having separate channels for the axial transport of the liquid and vapor phases of the working medium, and an artery supported generally concentrically within the evaporation section of the liquid channel for retaining liquid in the channel and assuring liquid feed to the evaporation section during periods of excessive heat transfer thereto.
2. Background of the Invention
Recent monogroove heat pipe developments have produced high performance heat pipes with tested heat transport performances in excess of 14,000 W-m, and theoretical capacities in excess of 25,000 W-m. These improvements represent an increase in heat transport capacity of better than two orders of magnitude over other currently existing heat pipes designed to operate at near ambient temperatures.
The basic monogroove heat pipe design incorporates two relatively large, independent axial channels, a larger one for vapor and a smaller one for liquid. These provide for handling the axial transport of the fluids (liquid and vapor) independently from the radial transfer of the heat, the latter being facilitated by liquid-conducting circumferential wall grooves in the vapor channel. A small capillary slot separates (interconnects) the otherwise independent channels and provides for the passage of fluid therebetween. The small slot sustains a high capillary pressure difference which, coupled with the minimized flow resistance provided by the two separate channels, results in the high axial heat transport capacity of the monogroove heat pipe design. The overall design also provides high evaporation and condensation film coefficients for the working fluid by means of the circumferential grooves in the walls of the vapor channel, while not interfering with the overall heat transport capability of the axial groove.
Such a monogroove heat pipe design has particular utility in zero-g environments, for example, such as for meeting the heat rejection requirements for large space platforms or space stations, and wherein capillary forces alone entirely control the working fluids in the heat pipe operation, no moving parts or auxiliary equipment being required.
As is recognized in this type of design, a continuous liquid flow path between the primary axial channel or groove and the circumferential wall grooves in the evaporation section of the vapor channel must be assured. This continuity must be maintained even with both groove menisci realistically depressed to reflect maximum heat flux conditions. One particular advantage of the monogroove heat pipe design is its inherent resistance to nucleate boiling within the axial liquid flow channel under high loads. In current designs, this is largely a consequence of separating the liquid channel and the heat input zone by locating the heat input zone at the top side of the vapor channel, opposite the liquid channel. Should gas bubbles form or become entrapped within the liquid channel, a particular advantage of the separate liquid and vapor channels is that such gas bubbles can readily be vented into the vapor channel through the common monogroove slot. A disadvantage is that the heat load usually has to be temporarily reduced to reprime the liquid channel.
It is to be expected that, in typical applications, periods of excessive heat transfer to the heat pipe will occur from time to time. Even without excessive heat transfer, it has been observed that loss of subcooling in the liquid channel in the evaporation section (caused, for example, by heat conducted to the liquid channel through the heat pipe walls) will limit heat transport capacity and cause high temperature gradients in the channel, largely due to vapor formation in the liquid channel. This results, in fact, in a substantial degradation of performance. It can be controlled to some extent by contouring the heat pipe walls to minimize such heat conduction, but such measures are limited where high-pressure fluids, such as ammonia, are used, since it can result in an unacceptably weak section subject to severe distortion under load.
A need therefore remains for a monogroove heat pipe which is resistant to superheating of the liquid channel in the evaporation section, and which will maintain and preserve the continuity of the fluid flow within the liquid channel to the evaporation section even during periods of excessive heat transfer thereto.